If a user configures a SerializationContextInitializer it is currently registered to the same SerializationContext as the persistent internal types. This means that it is possible for a user to reference internal protobuf message types in their .proto files. 

We should introduce the following:

1. A "user" SerializationContext that is used exclusively for user types configured via xml/service-loader/programmatically:
   User types are only registered with this context. The global and persistence contexts do not register these types.  If a type does not exist in the global/persistence context, then it's wrapped with a MarshallableUserObject and marshalling is delegated to the user marshaller. Previously this only occurred if a custom marshaller was defined as user types were added to the global/persistence context, but as this is no longer the case all user types are wrapped.
2. A USER_MARSHALLER created by MarshallerFactory, which if a custom marshaller is not defined creates a ProtoStream based marshaller that consumes the user SerializationContext, if a custom marshaller is defined then that is returned:
   This logic was previously in the PersistenceMarshallerImpl, but this allows us to inject the marshaller directly into various places in the code.
3. DelegatingUserMarshaller. A wrapper class to call the start/stop of the configured user marshaller and log the marshaller being used.